(1) Field of the Invention
The present invention relates to an electronic device, such as an image forming apparatus, loaded with a secondary battery, and to a power-source device used in the electronic device.
(2) Description of the Related Art
Some electronic devices use, in parallel, (i) power supplied from a commercial power source (external power source) and (ii) power supplied from a secondary battery such as a nickel hydride battery.
For example, the following MFP (Multiple Function Peripheral) device, a type of electronic device, has been proposed. That is to say, the MFP device includes a processing unit that includes: an image forming unit for executing an image forming operation; and an accepting unit including an external interface or the like for accepting a processing request (instruction) for the image forming unit. For the MFP device, a structure for switching the device between an operating state and a waiting state has been proposed, wherein, in the operating state, the image forming operation is executed.
Here, the waiting state is a state in which power is not supplied from the commercial power source (external power source) to the image forming unit and the accepting unit (including the external interface or the like), but power is supplied from the secondary battery to the accepting unit. Also, the operating state is a state in which power is supplied from the commercial power source to the image forming unit and the accepting unit.
When the MFP device does not execute the image forming operation, the MFP device is switched to the waiting state. This prevents the power from the commercial power source from being used in the waiting state, thus saving power consumption. Furthermore, since power is supplied from the secondary battery to the accepting unit in the waiting state, the accepting unit can accept a processing request for image formation even in the waiting state.
In such an image forming apparatus, when the capacity of the secondary battery decreases, an amount of power sufficient enough to make the accepting unit operable may not be supplied to the accepting unit, and the accepting unit including the external interface or the like may not accept a processing request for image formation in the waiting state. To prevent this in advance, a decrease in the capacity of the secondary battery may be detected and the secondary battery may be charged at an appropriate timing so that the secondary battery is recharged to the original capacity.
Patent Literature 1 (Japanese Patent Application Publication No. 2009-052975) discloses a structure for setting in advance a correspondence between the remaining capacity of the secondary battery (assembled battery) and the terminal voltage of the secondary battery such that the higher the internal resistance of the secondary battery is, the lower the terminal voltage is, and detecting the remaining capacity of the secondary battery based on the set correspondence.
Also, Patent Literature 2 (Japanese Patent Application Publication No. 2005-315597) discloses a structure for detecting the remaining capacity rate of the secondary battery with high accuracy by approximating the change in discharge voltage of the secondary battery from a full-charge state to a discharge end.
According to the structures of Patent Literatures 1 and 2, it is possible to detect the remaining capacity of the secondary battery with high accuracy, and charge the secondary battery at an appropriate timing when the remaining capacity of the secondary battery reaches a predetermined amount. This makes it possible to prevent the capacity of the secondary battery from decreasing in advance, and prevent the accepting unit from becoming unable to accept a processing request in the waiting state.
However, even if the remaining capacity of the secondary battery is detected with high accuracy as described in Patent Literatures 1 and 2, a so-called memory effect may occur in the secondary battery when an additional charge is repeatedly performed, wherein the additional charge is a charge that is performed at a timing when the remaining capacity of the secondary battery reaches a predetermined threshold. Note that the memory effect of the secondary battery is a phenomenon where, after a repetitive performance of the additional charge, the voltage of the secondary battery rapidly decreases at the timing when the remaining capacity of the secondary battery reaches the threshold.
When the memory effect occurs in the secondary battery, the external interface may not be operated appropriately since the voltage supplied from the secondary battery thereto decreases. Furthermore in that case, it may be erroneously judged that the secondary battery has come to the end of life, although in the actuality the secondary battery still has a usable amount of capacity.